The invention is directed to a method and apparatus for producing a metallic tube, which is equipped with at least one light waveguide.
Optical transmission elements, whose light waveguides are surrounded by a metallic cladding, are employed in cable technology, for example for high-voltage overhead lines or submarine cables. As warranted, the light waveguides can be additionally embedded into a filling compound in the inside of a metallic cladding or tube. In particular, the metallic cladding or tube serves the purpose of protecting the light waveguides against chemical, mechanical stresses and/or other disturbing influences, such as, for example, ambient moisture or hydrogen diffusion. For manufacturing such an optical transmission element, a metal band is shaped into a tube, and at least one light waveguide with or without the filling compound is introduced into the tube and the tube is then welded.
In practice, the manufacture of such a metal tube equipped with a light waveguide can be made more complicated for various reasons.
For example, difficulties can occur, particularly when welding the metallic tube. There is the risk, for example, that an admissibly high thermal stressing and, thus, damage to the materials or even a partial burning of the filling compound and/or of the plastic coating of the light waveguide will occur. In addition, the welding process is also particularly critical because hydrogen can be released due to local overheating. Such hydrogen can be released, for example, from humid ambient air during the melting of the edges of the tube to form the weld, or from the filling compound or from the plastic coating of the light waveguide, etc. If such hydrogen is entrained under certain conditions into the inside of a completely welded tube, which is tight on all sides, the hydrogen will remain in the tube and lead to an undesirable increase in the transmission attenuation of the light waveguides.
In U.S. Pat. No. 5,072,870, whose disclosure is incorporated herein by reference thereto and which claims priority from German Application 40 03 311, a metal band is already reversibly lengthened to form a tube before or during its shaping. To that end, the metal band is already heated with the assistance of a heating device before its actual shaping process. The optical fibers are introduced into the slotted tube in the region of the lengthened, slotted tube section being formed, to move together with the tube. Subsequently, the previously lengthened slotted tube is mechanically closed, is longitudinally welded with the assistance of a welding means and, finally, is pulled through a gauge plate for reducing the cross section of the tube. A clamping jaw haul-off serves the purpose of pulling the tube through the device. The tube is then subsequently strain-relieved and supplied to a supply reel. Problems can particularly occur in such a method in that the metal band is reversibly lengthened before the actual shaping event and must then traverse a number of shaping stages, such as, for example, roller tools, a welding means as well as a gauge plate until the shaping for the tube to be fabricated is ultimately completed. In particular, these subsequent shaping steps give rise to the risk that the previously set lengthening of the metal band is charged with disturbing quantities in an uncontrolled way or is partially or, eventually, potentially even entirely lost again. An exact setting of the excess length of the optical fibers compared to the metal tube is thus rendered more difficult in practice.